At present, development of LTE (Long Term Evolution) as a fast mobile communication method is underway. LTE uses a wide band of 20 MHz at a maximum, and adopts OFDMA (Orthogonal Frequency Division Multiple Access) and MIMO (Multiple Input Multiple Output) in down-link communication, and adopts SC-FDMA (Single-Carrier Frequency Division Multiple Access) in up-link communication. As a result, the LTE enables to achieve a transfer rate of 300 Mbps or higher at a maximum in down-link communication, and a transfer rate of 75 Mbps at a maximum in up-link communication.
LTE is a specialized communication method for packet communication. It is assumed that mobile communication terminals which can use LTE adopts the conventional communication methods, such as W-CDMA (Wideband Code Division Multiple Access) and cdma2000 1x for circuit-switched voice communication.
In this case, if a mobile communication terminal always stands by for both of a mobile communication system for voice communication and an LTE system, the current consumption in the normal standby of the mobile communication terminal increases, shortening the drive time of the terminal. If the mobile communication terminal stands by only for an LTE system, it cannot receive an incoming voice, and fails to effect a so-called “telephone” function. If the mobile communication terminal stands by only for a mobile communication system for voice communication, good use of the LTE's merit, full-time IP connection, cannot be made.
As a solution to those problems, the CS (Circuit Switched) Fallback function of the LTE system is described in “3GPP TS23.272 V8.6.0: Circuit Switched Fallback in Evolved Packet System; Stage 2” (hereinafter referred to as Non-patent Literature 1). In the CS Fallback function, a voice incoming notification from a mobile communication system for voice communication, such as W-CDMA or cdma2000 1x, is tunneled onto an LTE system to be transmitted to a mobile communication terminal. Then, the mobile communication terminal which has received the voice incoming notification performs a temporary disconnection (suspend) process on the LTE system, and performs circuit connecting process on the mobile communication system for voice communication to execute voice communication.
When voice communication is terminated later, the mobile communication terminal performs a circuit release process on the mobile communication system for voice communication, and performs a temporary-disconnection canceling (resume) process on the LTE system to return to standby for the LTE system. The same CS Fallback operation for the voice incoming process is carried out for a voice transmission process.
Since the mobile communication terminal in Non-patent Literature 1 normally stands by only for an LTE system, as mentioned above, the mobile communication terminal can have smaller current consumption than the type which stands by for both a mobile communication system for voice communication and LTE system.
In case of voice incoming, however, the mobile communication terminal in Non-patent Literature 1 needs to perform the temporary disconnection process for the LTE system and a process of acquiring the mobile communication system for voice communication while circuit connection is completed after reception of voice incoming notification from the LTE system. Therefore, the mobile communication terminal takes a longer time to actually start talking after reception of the voice incoming notification as compared with the mobile communication terminal which has finished the process of acquiring the mobile communication system for voice communication beforehand and is standing by in the standby state.
The same is true of the case of a transmission process; the mobile communication terminal in Non-patent Literature 1 needs to perform the temporary disconnection process for the LTE system and the process of acquiring the mobile communication system for voice communication while circuit connection is completed after the user performs the transmission process. Therefore, the mobile communication terminal in Non-patent Literature 1 takes a longer time to actually start talking after the user instructs voice transmission as compared with the mobile communication terminal which has finished the process of acquiring the mobile communication system for voice communication beforehand and is standing by in the standby state.